A styrene-acrylonitrile (SAN) resin which is a copolymer resin produced by polymerization of styrene and acrylonitrile is known to have good transparency, chemical resistance, rigidity or the like. Such a SAN resin is widely used in automobile interiors, household appliance housings and the like in combination with an acrylonitrile-butadiene-styrene (ABS) resin. In particular, the automobile interiors and the like are always exposed to high heat and chemical environments, thus requiring high heat resistance and chemical resistance.
In this regard, in order to impart high heat resistance to the SAN resin, alpha-methylstyrene monomers are generally incorporated, but more oligomers are produced and heat resistance is thus deteriorated when polymerization temperatures are decreased due to low depolymerization temperature of alpha-methylstyrene. For this reason, unlike typical SAN polymerization, polymerization is conducted at low polymerization temperatures. However, when the polymerization temperature is excessively low, there occur problems of deterioration in polymerization rate and polymerization conversion rate. In addition, when a reaction retention time is lengthened or an excess initiator is used so as to solve these problems, there occur problems of deterioration in molecular weight and production efficiency. As a result, overall physical properties including heat resistance and chemical resistance are deteriorated due to low molecular weight and more defects are disadvantageously generated upon product formation.